U.S. patent number 7,133,465 [Application Number 10/168,630] was granted by the patent office on 2006-11-07 for modified tomlinson-harashima precoding method circuit for infinite impulse response (iir) channels.
This patent grant is currently assigned to Inter-Tel, Inc.. Invention is credited to Michael Joseph McLaughlin.
United States Patent |
7,133,465 |
McLaughlin |
November 7, 2006 |
Modified Tomlinson-Harashima precoding method circuit for infinite
impulse response (IIR) channels
Abstract
A precoding circuit for minimizing distortion of an input signal
in a communication channel having a compound transfer function in
the Z-Domain:
H(z)=(1+a.sub.1.z.sup.-1+a.sub.2.z.sup.-2+a.sub.3.z.sup.31 3+ . . .
+a.sub.n.z.sup.-n)/(1+b.sub.1.z.sup.-+b.sub.2.z.sup.-2+b.sub.-3.z.sup-
.-3+ . . . +b.sub.p.z.sup.-p). A feedback circuit comprises a first
FIR filter and a transfer function corresponding to the inverse of
a feedforward part of a transfer function of the communication
channel. A feedforward circuit comprises a second FIR filter and a
transfer function corresponding to the inverse of a feedback part
of a transfer function of the communication channel. A first
subtracting circuit subtracts a feedback signal of the feedback
circuit from a feedforward signal of the feedforward circuit and
outputs a difference signal which is added to a transmit end signal
by a first adding circuit. The difference signal is also added to
the input signal by a second adding circuit, the output from the
second adding circuit is fed to a quantizing circuit which outputs
a value kM which is subtracted from the input signal by a second
subtracting circuit.
Inventors: |
McLaughlin; Michael Joseph
(Dublin, IE) |
Assignee: |
Inter-Tel, Inc. (Tempe,
AZ)
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Family
ID: |
11042181 |
Appl.
No.: |
10/168,630 |
Filed: |
December 22, 2000 |
PCT
Filed: |
December 22, 2000 |
PCT No.: |
PCT/IE00/00167 |
371(c)(1),(2),(4) Date: |
November 08, 2002 |
PCT
Pub. No.: |
WO01/48995 |
PCT
Pub. Date: |
July 05, 2001 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030123569 A1 |
Jul 3, 2003 |
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Foreign Application Priority Data
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Dec 23, 1999 [IE] |
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S991096 |
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Current U.S.
Class: |
375/296;
375/233 |
Current CPC
Class: |
H04L
25/03343 (20130101); H04L 25/497 (20130101) |
Current International
Class: |
H04L
25/03 (20060101); H03H 7/30 (20060101) |
Field of
Search: |
;375/219,229,232,233,257,295,296,377,285 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
M Sellers, et al. "Stabilized precoder with antenna diversity for
wireless LANs" IEEE Transactions on Consumer Electronics, IEEE
Inc., New York, US, vol. 45, No. 4, Nov. 1999, pp. 1169-1175,
XP002137332. cited by other .
G. D. Forney, Jr., et al. "Combined Equalization and Coding Using
Precoding", IEEE Communication Magazine IEEE Service Center.
Piscataway, N.J., US, vol. 29, No. 12, Dec. 1, 1991, pp. 25-34,
XP000287979. cited by other .
R Fischer et al.; "Signalformung Zur Begrenzung Der Dynamik Bei Der
Tomlinson-Harashima-Vercodierung" Vortrage Der ITG-Fachtagung,
Munchen, Oct. 26-28, 1994, Berlin, VDE Verlag, DE, vol. No. 130,
1994, pp. 457-466, XP000503821. cited by other.
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Primary Examiner: Tse; Young T.
Attorney, Agent or Firm: Whittington, Esq.; Michelle
Claims
The invention claimed is:
1. A precoding circuit for connecting to a communication channel
(21, 51, 61, 71) at a transmit end (22) thereof for minimising
distortion of an input signal of range +M/2 to -M/2 being
transmitted through the communication channel (21, 51, 61, 71)
between the transmit end (22) and a receive end (23) of the
communication channel (21, 51, 61, 71), whereby the distortion is
due to a transfer function of the communication channel (21, 51,
61, 71), the precoding circuit (20, 50, 60, 70) comprising: a
feedback circuit (27) connected to the transmit end of the
communication channel (21, 51, 61, 71), and having a transfer
function which is the inverse of a feedforward part of the transfer
function of the communication channel, a feedforward circuit (31)
connected to the transmit end of the communication channel (21, 51,
61, 71), and having a transfer function which is the inverse of a
feedback part of the transfer function of the communication channel
(21, 51, 61, 71,), a first circuit means (35) for subtracting a
feedback signal of the feedback circuit (27) from a feedforward
signal of the feedforward circuit (31), and for outputting a
difference signal, which is the difference of the feedback and
feedforward signals, a second circuit means (39) for outputting a
control signal of value kM derived from the difference signal and
the input signal, where k is an integer which is a positive,
negative or zero, and for selecting the value of k such that when
the control signal is subtracted from the input signal at the
transmit end (22) the value of the signal being outputted from the
precoding circuit to the communication channel (21, 51, 61, 71) is
minimised, and a third circuit means (40) for subtracting the
control signal from the input signal upstream of the feedback and
feedforward circuits (27, 31).
2. A precoding circuit as claimed in claim 1 characterised in that
the feedforward signal from the feedforward circuit (31) is applied
to the transmit end of the communication channel (21, 51, 61, 71)
upstream of a node (30) from which the feedback circuit (27)
derives its input from the transmit end of the communication
channel (21, 51, 61, 71).
3. A precoding circuit as claimed in claim 1 characterised in that
the feedforward circuit (31) is upstream of the feedback circuit
(27), or alternatively, the feedforward circuit (31) is downstream
of the feedback circuit (27), and a node (34) from which the
feedforward circuit derives its input from the transmit end (22) of
the communication channel (21, 51, 61, 71,), is downstream of the
feedback circuit (27).
4. A precoding circuit as claimed in claim 1 characterised in that
the first circuit means (35) comprises a first subtracting means
(35) for subtracting the feedback signal from the feedforward
signal and for outputting the difference signal, and preferably, a
first adding means (37) is provided for adding the difference
signal from the first circuit means (35) to the signal in the
transmit end of the communication channel (21, 51, 61, 71), and
advantageously, the value of k of the control signal is selected by
the second circuit means (39) such that when the control signal is
subtracted from the input signal the value of the signal at the
output from the precoding circuit (20, 50, 60, 70), as it is fed to
the communication channel (21, 51, 61, 71), is within the range
+M/2 to -M/2.
5. A precoding circuit as claimed in claim 1 characterised in that
the difference signal and the input signal are added to provide an
intermediate signal, and the intermediate signal is fed to the
second circuit means (39), and the control signal is derived by the
second circuit means (39) from the intermediate signal, and
preferably, a second adding means (38) is provided for adding the
input signal and the difference signal from the first circuit means
(35) for providing the intermediate signal, and preferably, the
second circuit means (39) comprises a quantising circuit (39) for
deriving the control signal from the intermediate signal.
6. A precoding circuit as claimed in claim 1 characterised in that
the third circuit means (40) comprises a second subtracting means
(40) for subtracting the control signal from the input signal, and
preferably, the difference signal from the first circuit means (35)
is applied to the transmit end of the communication channel (21,
51, 61, 71) intermediate respective nodes (30, 34) from which the
feedback circuit (27) and the feedforward circuit (31) derive their
respective inputs from the transmit end of the communication
channel (21, 51, 61, 71).
7. A precoding circuit as claimed in claim 1 characterised in that
the impulse response of the communication channel (21, 51, 61, 71)
is finite, and preferably, the feedforward part of the transfer
function of the communication channel (21, 51, 61, 71) can be
broadly expressed in the Z-Domain by the equation:
H(z)=(1+a.sub.1.z.sup.-1+a.sub.2.z.sup.-2+a.sub.3.z.sup.-3+ . . .
+a.sub.n.z.sup.-n) where a is a filter coefficient multiplier, and
z is a time of n time-steps.
8. A precoding circuit as claimed in claim 1 characterised in that
the impulse response of the communication circuit (21, 51, 61, 71)
is infinite, and preferably, the feedback part of the transfer
function of the communication channel (21, 51, 61, 71) can be
broadly expressed in the Z-Domain by the equation: .times.
##EQU00023## where b is a filter coefficient multiplier, and z is a
time of p time-steps.
9. A precoding circuit as claimed in claim 1 characterised in that
the compound transfer function of the communication channel (21,
51, 61, 71) can be broadly expressed in the Z-Domain by the
equation .times. ##EQU00024## where a and b are filter coefficient
multipliers, and z is a time of n and p time-steps, and preferably,
the transfer function of the feedback circuit (27) can be broadly
expressed in the Z-Domain by the equation: .function. ##EQU00025##
where a is a filter coefficient multiplier, and z is a time of n
time-steps, and advantageously, the transfer function of the
feedforward circuit (31) can be broadly expressed in the Z-Domain
by the equation:
H(z)=(1+b.sub.1.z.sup.-1+b.sub.2.z.sup.-2+b.sub.3.z.sup.-3+ . . .
+b.sub.p.z.sup.-p) where b is a filter coefficient multiplier, and
z is a time of p time-steps.
10. A precoding circuit as claimed in claim 1 characterised in that
the precoding circuit is adapted for use with a subset of
communication channels where the feedforward part of the transfer
function is minimum phase.
11. A precoding circuit as claimed in claim 1 characterised in that
a gain circuit means (52) is provided at the transmit end of the
communication channel (21, 51, 61, 71), downstream of the precoding
circuit (20, 50, 60, 70) for compensating for a first coefficient
of the feedforward part of the transfer function of the
communication channel (21, 51, 61, 71), and preferably, the
precoding circuit (20, 50, 60, 70) is for equalising a subset of
communication channels where the first coefficient of the
feedforward part of the transfer function is equal to one, and
advantageously, a third adding means (41) is provided at the
receive end (23) of the communication channel (21, 51, 61, 71) for
adding a value kM to the signal at the receive end (23) of the
communication channel (21, 51, 61, 71) for facilitating recovery of
the input signal at the receive end (23), the value of kM
corresponding to the value of kM subtracted from the input signal
at the transmit end (22) of the communication channel (21, 51, 61,
71).
12. A precoding circuit as claimed in claim 1 characterised in that
the precoding circuit is adapted for use with a non-linear
constellation, and a first converting means (72) is provided for
converting the signal at the transmit end of the communication
channel (21, 51, 61, 71), from being linear to being non-linear,
and preferably, the first converting means (72) is located after
the third circuit means, and preferably, a second converting means
(73) is provided at the receive end of the communication channel
(21, 51, 61, 71) for converting the received signal at the receive
end from non-linear to linear.
13. A communication circuit comprising the precoding circuit (20,
50, 60, 70) as claimed in claim 1.
14. A method for minimising distortion of a signal of range +M/2 to
-M/2 being transmitted through a communication channel between a
transmit end and a receive end of the communication channel,
whereby the distortion is due to a transfer function of the
communication channel, the method comprising the steps of: feeding
back the signal at the transmit end of the communication channel
through a feedback circuit having a transfer function which is the
inverse of a feedforward part of the transfer function of the
communication channel, feeding forward the signal at the transmit
end of the communication channel through a feedforward circuit
having a transfer function which is the inverse of a feedback part
of the transfer function of the communication channel, subtracting
a feedback signal of the feedback circuit from a feedforward signal
of the feedforward circuit to provide a difference signal, deriving
a control signal of value kM from the difference signal and an
input signal, where k is an integer which is a positive, negative
or zero, the value of k being selected such that when the control
signal of value kM is subtracted from the input signal at the
transmit end, the value of a signal being outputted from a
precoding circuit to the communication channel is minimised, and
subtracting the control signal of value kM from the input signal
upstream of the feedback and feedforward circuits.
15. A method as claimed in claim 14 characterised in that the
feedforward signal from the feedforward circuit is applied to the
transmit end of the communication channel upstream of a node from
which the feedback circuit derives its input from the transmit end
of the communication channel.
16. A method as claimed in claim 14 characterised in that the
feedforward circuit is upstream of the feedback circuit.
17. A method as claimed in claim 14 characterised in that the
feedback and feedforward signals are applied to the transmit end of
the communication channel intermediate respective nodes from which
the feedback circuit and the feedforward circuit derive their
respective inputs at the transmit end of the communication channel,
alternatively, the feedforward circuit is downstream of the
feedback circuit, and a node from which the feedforward circuit
derives input from the transmit end of the communication channel is
downstream of the feedback circuit.
18. A method as claimed in claim 14 characterised in that the value
of k of the control signal is selected such that when the control
signal is subtracted from the input signal at the transmit end of
the communication channel, the value of the signal being outputted
from the precoding circuit to the communication channel at the
transmit end thereof is within the range +M/2 to -M/2, and
preferably, the difference signal and the input signal are added to
provide an intermediate signal, and the control signal of value kM
is derived from the intermediate signal.
19. A method as claimed in claim 14 characterised in that the
impulse response of the communication channel is finite, and
preferably, the feedforward part of the transfer function of the
communication channel can be broadly expressed in the Z-Domain by
the equation:
H(z)=(1+a.sub.1.z.sup.-1+a.sub.2.z.sup.-2+a.sub.3.z.sup.-3+ . . .
+a.sub.n.z.sup.-n) where a is a filter coefficient multiplier, and
z is a time of n time-steps.
20. A method as claimed in claim 14 the impulse response of the
communication channel is infinite, and preferably, the feedback
part of the transfer function of the communication channel can be
broadly expressed in the Z-Domain by the equation: .function.
##EQU00026## where b is a filter coefficient multiplier, and z is a
time of a p time-steps, and preferably, the compound transfer
function of the communication channel can be broadly expressed in
the Z-Domain by the equation: .function. ##EQU00027## where a and b
are filter coefficient multipliers, and z is a time of n and p
time-steps, and advantageously, the transfer function of the
feedback circuit can be broadly expressed in the Z-Domain by the
equation: .function. ##EQU00028## where a is a filter coefficient
multiplier, and z is a time of n time-steps, and preferably, the
transfer function of the feedforward circuit can be broadly
expressed in the Z-Domain by the equation:
H(z)=(1+b.sub.1.z.sup.-1+b.sub.2.z.sup.-2+b.sub.3.z.sup.-3+ . . .
+b.sub.p.z.sup.-p) where b is a filter coefficient multiplier, and
z is a time of p time-steps.
21. A method as claimed in claim 14 characterised in that the
method is adapted for use with a subset of communication channels
where the feedforward part of the transfer function is minimum
phase, and preferably, the signal being fed to the communication
channel at the transmit end thereof is multiplied by a gain factor
for compensating for a first coefficient of the feedforward part of
the transfer function of the communication channel, and
advantageously, the method is for equalising a subset of
communication channels where the first coefficient of the
feedforward part of the transfer function is equal to one, and
preferably, the method further comprises the step of adding a value
of kM to a signal at the receive end of the communication channel
for recovering the input signal at the receive end, the value of kM
corresponding to the value of kM subtracted from the input signal
at the transmit end of the communication channel.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a method and to a precoding
circuit for minimising distortion of a signal being transmitted
through a communication channel whereby the distortion is due to
the transfer function of the communication channel. In particular,
the invention relates to a method and a precoding circuit for
pre-equalising a subset of communication channels where the first
coefficient of the feedforward part of the transfer function is
equal to zero, although the invention is not so limited.
(2) Description of Related Art
It is known to provide a Tomlinson-Harashima precoding circuit and
method for pre-equalisation of a communication channel whose
impulse response is finite and known, in other words, for
communication channels with a feedforward transfer function which
can be expressed by the following equation in the Z-Domain:
H(z)=(1+a.sub.1.z.sup.-1+a.sub.2.z.sup.-2+a.sub.3.z.sup.-3+ . . .
+a.sub.n.z.sup.-n)
The Tomlinson-Harashima precoding method and circuit is disclosed
in "New Automatic Equaliser Employing Modulo Arithmetic",
Electronic Letters, Vol. 7, Nos. 5/6, Mar. 25, 1971, pp. 138 139 by
Tomlinson, M and Digital Communication, 2.sup.nd edition, by Edward
A. Lee and David G. Messerschmidt, Kluwer Academic Publishers. FIG.
1 illustrates a block representation of such a precoding circuit
according to Tomlinson-Harashima. An input data signal in the
range
.times..times..times. ##EQU00001## is applied to a communication
channel at the transmit end and is received at the receive end. In
the Tomlinson-Harashima precoding circuit a feedback circuit is
provided at the transmit end of the communication channel, and the
feedback circuit has the transfer function which is the inverse of
the feedforward part of the transfer function of the communication
channel. A feedback signal from the feedback circuit is subtracted
from the signal at the transmit end of the communication channel. A
modulo operator is provided in the communication channel between
the input and the output of the feedback circuit whereby a function
kM is subtracted from the signal at the transmit end of the
communication channel. The term k is an integer which may be
positive, negative or zero and is chosen so that the output of the
modulo operator is in the range
.times..times..times..times. ##EQU00002## . A similar modulo
operator is provided at the receive end of the communication
channel through which the signal at the receive end is passed for
facilitating the recovery of the input data at the receive end.
However, the Tomlinson-Harashima precoding circuit and method while
it is suitable for pre-equalisation of a communication channel, it
is only suitable for dealing with the feedforward part of the
transfer function of the communication channel.
There is therefore a need for a method and a precoding circuit for
pre-equalisation of a communication channel for minimising
distortion of an input signal being transmitted in the
communication channel which takes account of both the feedforward
and feedback parts of the transfer function of the communication
channel.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed towards providing such a method
and a precoding circuit.
According to the invention there is provided a precoding circuit
for connecting to a communication channel at a transmit end thereof
for minimising distortion of an input signal of range
.times..times..times. ##EQU00003## being transmitted through the
communication channel between the transmit end and a receive end of
the communication channel, whereby the distortion is due to the
transfer function of the communication channel, the precoding
circuit comprising: a feedback circuit connected to the transmit
end of the communication channel, and having a transfer function
which is the inverse of the feedforward part of the transfer
function of the communication channel, a feedforward circuit
connected to the transmit end of the communication channel, and
having a transfer function which is the inverse of the feedback
part of the transfer function of the communication channel, a first
circuit means for subtracting the feedback signal of the feedback
circuit from the feedforward signal of the feedforward circuit, and
for outputting a difference signal, which is the difference of the
feedback and feedforward signals, a second circuit means for
outputting a control signal of value kM derived from the difference
signal and the input signal, where k is an integer and may be
positive, negative or zero, and for selecting the value of k such
that when the control signal is subtracted from the input signal at
the transmit end the value of the signal being outputted from the
precoding circuit to the communication channel is minimised, and a
third circuit means for subtracting the control signal from the
input signal upstream of the feedback and feedforward circuits.
In one embodiment of the invention the feedforward signal from the
feedforward circuit is applied to the transmit end of the
communication channel upstream of a node from which the feedback
circuit derives its input from the transmit end of the
communication channel.
In another embodiment of the invention the feedforward circuit is
upstream of the feedback circuit.
Alternatively, the feedforward circuit is downstream of the
feedback circuit, and a node from which the feedforward circuit
derives its input from the transmit end of the communication
channel is downstream of the feedback circuit.
In a further embodiment of the invention the first circuit means
comprises a first subtracting means for subtracting the feedback
signal from the feedforward signal and for outputting the
difference signal.
In one embodiment of the invention a first adding means is provided
for adding the difference signal from the first circuit means to
the signal in the transmit end of the communication channel.
In another embodiment of the invention the value of k of the
control signal is selected by the second circuit means such that
when the control signal is subtracted from the input signal the
value of the signal at the output from the precoding circuit as it
is fed to the communication channel is within the range
.times..times..times..times. ##EQU00004##
In a further embodiment of the invention the difference signal and
the input signal are added to provide an intermediate signal, and
the intermediate signal is fed to the second circuit means, and the
control signal is derived by the second circuit means from the
intermediate signal.
In a still further embodiment of the invention a second adding
means is provided for adding the input signal and the difference
signal from the first subtracting means for providing the
intermediate signal.
In one embodiment of the invention the second circuit means
comprises a quantising circuit for deriving the control signal from
the intermediate signal.
In one embodiment of the invention the third circuit means
comprises a second subtracting means for subtracting the control
signal from the input signal.
Preferably, the difference signal from the first subtracting means
is applied to the transmit end of the communication channel
intermediate the respective nodes from which the feedback circuit
and the feedforward circuit derive their respective inputs from the
transmit end of the communication channel.
In one embodiment the feedback circuit is an infinite impulse
response structure.
Preferably, the feedback circuit comprises a first finite impulse
response filter.
In one embodiment of the invention the feedforward circuit is a
finite impulse response structure.
Preferably, the feedforward circuit comprises a second finite
impulse response filter.
Advantageously, the first coefficient of the second finite impulse
response filter is one.
In one embodiment of the invention the impulse response of the
communication channel is finite.
In one embodiment of the invention the feedforward part of the
transfer function of the communication channel can be broadly
expressed in the Z-Domain by the equation:
H(z)=(1+a.sub.1.z.sup.-1+a.sub.2.z.sup.-2+a.sub.3.z.sup.-3+ . . .
+a.sub.n.z.sup.-n)
In another embodiment of the invention the impulse response of the
communication circuit is infinite.
In another embodiment of the invention the feedback part of the
transfer function of the communication channel can be broadly
expressed in the Z-Domain by the equation:
.function..times..times. ##EQU00005##
In a further embodiment of the invention the compound transfer
function of the communication channel can be broadly expressed in
the Z-Domain by the equation:
.function..times..times..times..times. ##EQU00006##
In one embodiment of the invention the transfer function of the
feedback circuit can be broadly expressed in the Z-Domain by the
equation:
.function..times..times. ##EQU00007##
In one embodiment of the invention the transfer function of the
feedforward circuit can be broadly expressed in the Z-Domain by the
equation:
H(z)=(1+b.sub.1.z.sup.-1+b.sub.2.z.sup.-2+b.sub.3.z.sup.-3+ . . .
+b.sub.p.z.sup.-p)
In one embodiment of the invention the precoding circuit is adapted
for use with a subset of communication channels where the
feedforward part of the transfer function is minimum phase.
In one embodiment of the invention a gain circuit means is provided
at the transmit end of the communication channel downstream of the
precoding circuit for compensating for the first coefficient of the
feedforward part of the transfer function of the communication
channel.
In one embodiment of the invention the precoding circuit is for
equalising a subset of communication channels where the first
coefficient of the feedforward part of the transfer function is
equal to one.
In another embodiment of the invention a third adding means is
provided at the receive end of the communication channel for adding
a value kM to the signal at the receive end of the communication
channel for facilitating recovery of the input signal at the
receive end, the value of kM corresponding to the value of kM
subtracted from the signal at the transmit end of the communication
channel.
In one embodiment of the invention the precoding circuit is adapted
for use with a non-linear constellation, and a first converting
means is provided for converting the signal at the transmit end of
the communication channel from being linear to being non-linear.
Preferably, the first converting means is located after the third
circuit means. Advantageously, a second converting means is
provided at the receive end of the communication channel for
converting the received signal at the receive end from non-linear
to linear.
Further, the invention provides a communication circuit comprising
a precoding circuit according to the invention.
Additionally, the invention provides a method for minimising
distortion of a signal of range
.times..times..times. ##EQU00008## being transmitted through a
communication channel between a transmit end and a receive end of
the communication channel, whereby the distortion is due to the
transfer function of the communication channel, the method
comprising the steps of: feeding back the signal at the transmit
end of the communication channel through a feedback circuit having
a transfer function which is the inverse of the feedforward part of
the transfer function of the communication channel, feeding forward
the signal at the transmit end of the communication channel through
a feedforward circuit having a transfer function which is the
inverse of the feedback part of the transfer function of the
communication channel, subtracting the feedback signal of the
feedback circuit from the feedforward signal of the feedforward
circuit to provide a difference signal, deriving a control signal
of value kM from the difference signal and the input signal, where
k is an integer and may be positive, negative or zero, the value of
k being selected such that when the control signal of value kM is
subtracted from the input signal at the transmit end, the value of
the signal being outputted from the precoding circuit to the
communication channel is minimised, and subtracting the control
signal of value kM from the input signal upstream of the feedback
and feedforward circuits.
In one embodiment of the invention the feedforward signal from the
feedforward circuit is applied to the transmit end of the
communication channel upstream of a node from which the feedback
circuit derives its input from the transmit end of the
communication channel.
In another embodiment of the invention the feedforward circuit is
upstream of the feedback circuit.
In a further embodiment of the invention the feedback and
feedforward signals are applied to the transmit end of the
communication channel intermediate respective nodes from which the
feedback circuit and the feedforward circuit derive their
respective inputs at the transmit end of the communication
channel.
Alternatively, the feedforward circuit is downstream of the
feedback circuit, and a node from which the feedforward circuit
derives its input from the transmit end of the communication
channel is downstream of the feedback circuit.
In one embodiment of the invention the value of k of the control
signal is selected such that when the control signal is subtracted
from the input signal at the transmit end of the communication
channel, the value of the signal being outputted from the precoding
circuit to the communication channel at the transmit end thereof is
within the range
.times..times..times. ##EQU00009##
In another embodiment of the invention the difference signal and
the input signal are added to provide an intermediate signal, and
the control signal of value kM is derived from the intermediate
signal.
In one embodiment of the invention the impulse response of the
communication channel is finite.
In another embodiment of the invention the feedforward part of the
transfer function of the communication channel can be broadly
expressed in the Z-Domain by the equation:
H(z)=(1+a.sub.1.z.sup.-1+a.sub.2.z.sup.-2+a.sub.3.z.sup.-3+ . . .
+a.sub.n.z.sup.-n)
In a further embodiment of the invention the feedback part of the
transfer function of the communication channel can be broadly
expressed in the Z-Domain by the equation:
.times. ##EQU00010##
In a still further embodiment of the invention the compound
transfer function of the communication channel can be broadly
expressed in the Z-Domain by the equation:
.times. ##EQU00011##
In one embodiment of the invention the transfer function of the
feedback circuit can be broadly expressed in the Z-Domain by the
equation:
.times. ##EQU00012##
In another embodiment of the invention the transfer function of the
feedforward circuit can be broadly expressed in the Z-Domain by the
equation:
H(z)=(1+b.sub.1.z.sup.-1+b.sub.2.z.sup.-2+b.sub.3.z.sup.-3+ . . .
+b.sub.p.z.sup.-p)
In one embodiment of the invention the method is adapted for use
with a subset of communication channels where the feedforward part
of the transfer function is minimum phase.
In another embodiment of the invention the signal being fed to the
communication channel at the transmit end thereof is multiplied by
a gain factor for compensating for the first coefficient of the
feedforward part of the transfer function of the communication
channel.
In one embodiment of the invention the method is for equalising a
subset of communication channels where the first coefficient of the
feedforward part of the transfer function is equal to one.
In another embodiment of the invention the method further comprises
the step of adding a value of kM to the signal at the receive end
of the communication channel for recovering the input signal at the
receive end, the value of-kM corresponding to the value of kM
subtracted from the signal at the transmit end of the communication
channel.
In one embodiment of the invention the method is adapted for use
with a non-linear constellation, and the signal at the transmit end
of the communication channel is converted from being linear to
being non-linear. Preferably, the signal at the transmit end of the
communication channel is converted from being linear to being
non-linear immediately after the control signal has been subtracted
from the input signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more clearly understood from the following
description of some preferred embodiments thereof, which are given
by way of example only, with reference to the accompanying drawings
in which:
FIG. 1 is a block representation of a Tomlinson-Harashima precoding
circuit according to the prior art for pre-equalisation of a
communication channel,
FIG. 2 is a more detailed circuit diagram of the
Tomlinson-Harashima prior art precoding circuit of FIG. 1,
FIG. 3 is an alternative circuit diagram of the Tomlinson-Harashima
prior art precoding circuit of FIG. 1,
FIG. 4 is a block representation of a precoding circuit according
to the invention for minimising distortion of an input signal being
transmitted in a communication channel between a transmit end and a
receive end,
FIG. 5 is a more detailed circuit diagram of the precoding circuit
of FIG. 4,
FIG. 6 is a block representation of a precoding circuit according
to another embodiment of the invention for minimising distortion in
an input signal being transmitted in a communication channel
between a transmit end and a receive end,
FIG. 7 is a block representation of a precoding circuit according
to a further embodiment of the invention for minimising distortion
in an input signal being transmitted in a communication channel
between a transmit end and a receive end, and
FIG. 8 is a block representation of a precoding circuit according
to a still further embodiment of the invention for minimising
distortion in an input signal being transmitted in a communication
channel between a transmit end and a receive end.
DETAILED DESCRIPTION
Referring to the drawings and initially to FIGS. 1 and 2, as
discussed above, a Tomlinson-Harashima precoding circuit according
to the prior art is illustrated and indicated generally by the
reference numeral 1 for minimising distortion of a signal through a
communication channel 2. The communication channel 2 has a finite
impulse response which is known and a feedforward part of its
transfer function which can be broadly expressed by the following
equation in the Z-Domain:
H(z)=(1+a.sub.1.z.sup.-1+a.sub.2.z.sup.-2+a.sub.3.z.sup.-3+ . . .
+a.sub.n.z.sup.-n)
An input data signal having a data range of
.times..times..times. ##EQU00013## to be transmitted through the
communication channel 2 is inputted at a transmit end 4 of the
communication channel 2. The input data is recovered from the
communication channel 2 at a receive end 5 of the communication
channel 2. The Tomlinson-Harashima precoding circuit 1 is provided
at the transmit end 4 and comprises a feedback circuit 6 which has
an infinite impulse response which is the inverse of the
feedforward part of the transfer function of the communication
channel 2. The input to the feedback circuit 6 is derived from a
node 10 at the transmit end 4 of the communication channel 2. The
feedback circuit 6 comprises an infinite impulse response filter 8
which comprises a plurality of one sample delay elements 11 and
coefficient multipliers 12. This provides the feedback circuit 6
with a transfer function which can be broadly represented in the
Z-Domain by the equation:
.times. ##EQU00014##
The feedback signal on an output line 9 from the feedback circuit 6
is subtracted from the data input signal at the transmit end 4, and
the difference of the input data signal and the feedback signal is
applied to a modulo operator 14. The modulo operator 14 subtracts a
value kM from the input to the modulo operator 14, where k is an
integer which may be either positive, negative or zero. The integer
k is selected so that the output from the modulo operator 14 is
within the range
.times..times..times. ##EQU00015## . Thus, the output of the modulo
M operator which is fed at the transmit end 4 into the
communication channel 2 is the pre-equalised version of the input
data signal. Since the transfer function of the feedback circuit 6
is the inverse of the feedforward transfer function of the
communication channel, by passing the signal at the receive end 5
of the communication channel 2 through an equivalent modulo
operator 15 at the receive end 5 and repeating the modulo operation
on the received signal at the receive end 5, the original input
data signal is recovered from the modulo M operator 15 at the
receive end 5. The construction and operation of the
Tomlinson-Harashima precoding circuit of FIGS. 1 and 2 will be well
known to those skilled in the art.
FIG. 3 illustrates an equivalent precoding circuit to the
Tomlinson-Harashima circuit of FIGS. 1 and 2, and similar
components are identified by the same reference numerals. In this
circuit, the function kM where k is any integer, positive, negative
or zero is subtracted from the input data signal at the transmit
end 4 upstream of the feedback circuit 6. The value of k is again
selected so that the transmit signal being transmitted at the node
10 at the transmit end 4 of the communication channel 2 is within
the range
.times..times..times. ##EQU00016## . In order to recover the input
data signal at the receive end 5, a corresponding value kM is added
to the received signal at the receive end 5 of the communication
channel 2 and the input data signal is recovered. Thus, it will be
apparent to those skilled in the art that the precoding circuit of
FIG. 3 is equivalent to the Tomlinson-Harashima precoding circuit
of FIGS. 1 and 2.
However, in all cases the prior art precoders are suitable only for
dealing with the feedforward part of the transfer function of the
communication channel. They do not deal with the feedback part of
the transfer function of the communication channel.
Referring now to FIGS. 4 and 5, a precoding circuit according to
the invention, which is indicated generally by the reference
numeral 20, for minimising distortion of an input data signal in a
telecommunication communication channel 21, between a transmit end
22 and a receive end 23 resulting from both the feedforward and
feedback parts of the transfer function of the communication
channel 21 will now be described. The precoding circuit 20 is
ideally suited for pre-equalising a subset of communication
channels where the first coefficient of the feedforward transfer
function is one. The communication channel 21 has an impulse
response which is infinite and known, and a feedforward part of its
transfer function which can be broadly represented in the Z-Domain
by the following equation:
H(z)=(1+a.sub.1.z.sup.-1+a.sub.2.z.sup.-2+a.sub.3.z.sup.-3+ . . .
+a.sub.n.z.sup.-n) and a feedback part of its transfer function
which can be broadly represented in the Z-Domain by the following
equation:
.times. ##EQU00017##
Accordingly, the compound transfer function of the communication
channel 21 can be broadly represented in the Z-Domain by the
following equation:
.times. ##EQU00018##
The circuit 20 comprises a feedback circuit 27 which is connected
at the transmit end 22 of the communication channel 21. The
feedback circuit 27 is an infinite impulse response structure, and
comprises a first finite impulse filter 28, which provides the
feedback circuit 27 with a transfer function which is the inverse
of the feedforward part of the transfer function of the
communication channel 21. A feedback signal from the feedback
circuit 27 is provided on an output line 29 from the first filter
28. The input to the feedback circuit is derived from a node 30 at
the transmit end 22 of the communication channel 21.
A feedforward circuit 31 is connected at the transmit end 22 to the
communication channel 21 upstream of the feedback circuit 27. The
feedforward circuit 31 is a finite response structure, and
comprises a second finite impulse response filter 32, the transfer
function of which is the inverse of the feedback part of the
transfer function of the communication channel 21. A feedforward
signal from the feedforward circuit 31 is provided on an output
line 33 from the second filter 32. The input to the feedforward
circuit 31 is derived from a node 34 at the transmit end 22 of the
communication channel 21.
The first filter 28 comprises a plurality of one sample delay
elements and coefficient multipliers, and thus provide the feedback
circuit 27 with a transfer function which is the inverse of the
feedforward part of the transfer function of the communication
channel 21. Thus, the transfer function of the feedback circuit 27
can be broadly represented in the Z-Domain by the equation:
.times. ##EQU00019##
The second filter 32 comprises a plurality of one sample delay
elements and coefficient multipliers, and thus provides the
feedforward circuit 31 with a transfer function which is the
inverse of the feedback part of the transfer function of the
communication channel 21, and can be represented in the Z-Domain by
the equation:
H(z)=(1+b.sub.1.z.sup.-1+b.sub.2.z.sup.-2+b.sub.3.z.sup.-3+ . . .
+b.sub.p.z.sup.-p)
A first circuit means comprising a first subtracting means, namely,
a first subtracting circuit 35, subtracts the feedback signal on
the output line 29 of the feedback circuit 27 from the feedforward
signal on the output line 33 of the feedforward circuit 31, and a
difference signal from the first subtracting circuit 35 is fed to a
first adding means, namely, a first adding circuit 37. The first
adding circuit 37 is provided in the transmit end of the
communication channel 21, and adds the difference signal from the
first subtracting circuit 35 to the transmit signal in the transmit
end 22 of the communication channel 21.
A second adding means, namely, a second adding circuit 38 adds the
input data signal with the difference signal from the first
subtracting circuit 35, and outputs an intermediate signal which is
the sum of the input data signal and the difference signal to a
second circuit means which comprises a quantising circuit 39. The
quantising circuit 39 outputs a control signal of value kM which is
derived from the intermediate signal. The control signal is
subtracted from the data input signal by a third circuit means,
namely, a second subtracting circuit 40. The term k selected by the
quantising circuit 39 is an integer, which may be positive,
negative or zero, but is selected to be of sign and value such that
the transmit signal at the node 30 at the output end of the
precoding circuit 20 is within the range
.times..times. ##EQU00020##
A third adding means, namely, a third adding circuit 41, is
provided at the receive end 23 of the communication channel 21 for
adding a corresponding value of kM to the receive signal at the
receive end 23 of the communication channel 21 so that the
recovered signal appearing on an output 43 of the third adding
circuit 41 corresponds with the input data signal which is applied
at the transmit end 22. Accordingly, since the feedback and
feedforward circuits 27 and 31, respectively, have respective
transfer functions which are the inverse of the feedforward and
feedback parts of the transfer function, respectively, of the
communication channel 21, the respective feedforward and feedback
parts of the transfer function of the communication channel 21 are
compensated for in the transmit signal at the output of the
precoding circuit 20, in other words at the node 30 to be
transmitted through the communication channel 21. By use of the
quantising circuit 39 for outputting the control signal of value kM
and for selecting k so that the value of kM is such that the
transmit signal at the node 30 at the transmit end 22 of the
communication channel 21 is always within the range +M/2 to -M/2
there is no danger of the combined feedback and feedforward
circuits going into an infinite loop. By summing a value equivalent
to the value of kM to the receive signal at the receive end 23 of
the communication channel 21, the subtraction of the control signal
of value kM from the input signal at the transmit end 22 of the
communication channel 21 is compensated for, and accordingly, the
signal appearing at the output terminal 43 at the receive end 23
corresponds with the input data signal inputted to the
communication channel 21 at the transmit end 22.
Referring now to FIG. 6, a precoding circuit 50 also according to
the invention for minimising distortion of an input data signal
being transmitted through a communication channel 51 as a result of
the feedforward and feedback part of the transfer function of the
communication channel 51 is illustrated. The precoding circuit 50
is substantially similar to the precoding circuit 20 and similar
components are identified by the same reference numerals. The main
difference between the precoding circuit 50 and the precoding
circuit 20 is that a gain means provided by a multiplying circuit
52 is provided between the precoding circuit 50 and the
communication channel 51 for compensating for the first coefficient
of the feedforward part of the transfer function of the
communication channel 51. In this embodiment of the invention the
compound transfer function of the communication channel 51 in the
Z-Domain can be broadly represented by the following equation:
.function..function..function. ##EQU00021## where g is the
gain.
It is desirable that the transfer function A(z) of the compound
transfer function is derived in the preceding circuit 50 by a
minimum phase filter, and this is achieved by transforming the
feedforward circuit 31 into its equivalent amplitude response
minimum phase equivalent. However, this gives an incorrect phase
response, and thus, an all-pass phase corrector (not shown) is
required after the node 30. The multiplying circuit 52 multiplies
the signal downstream of the node 30 by an appropriate gain factor
g. The product signal from the multiplying circuit 52 is then
transmitted through the communication channel 51.
Referring now to FIG. 7, there is illustrated another preceding
circuit also according to the invention, indicated generally by the
reference numeral 60, for minimizing distortion of an input signal
in a communication channel 61 which is similar to the communication
channel 21. The precoding circuit 60 is somewhat similar to the
preceding circuit 20 and similar components are identified by the
same reference numerals. In this embodiment of the invention the
feedback circuit 27 is provided upstream of the feedforward circuit
31. A subtracting circuit 62 subtracts the feedback signal on the
output line 29 of the feedback circuit 27 from the signal in the
transmit end 22 of the communication channel 61. An adding circuit
63 downstream of the subtracting circuit 62 adds the feedforward
signal on the output line 33 of feedforward circuit 31 to the
signal in the transmit end 22 of the communication channel 61. The
first subtracting circuit 64 subtracts the feedback signal on the
output line 29 from the feedforward signal on the output line 33
from the respective feedback circuit 27 and the feedforward circuit
31, respectively, and the difference signal from the first
subtracting circuit 64 is added to the input data signal in the
second adding circuit 38. Otherwise, the precoding circuit 60 is
similar to the precoding circuit 20 and its operation is likewise
similar. However, in certain instances, additional circuitry (not
shown) may be required in order to prevent the output from the
precoding circuit 60 which is fed to the communication circuit 61
going to infinity.
Referring now to FIG. 8, there is illustrated a precoding circuit
according to a further embodiment of the invention which is
indicated generally by the reference numeral 70. The precoding
circuit in this embodiment of the invention is suitable for use
with non-linear constellations. The precoding circuit 70 is
substantially similar to the precoding circuit 20 described with
reference to FIGS. 4 and 5, and similar components are identified
by the same reference numerals. The main difference between the
precoding circuit 70 and the precoding circuit 20 is that a first
converting means comprising a first conversion circuit 72 is
provided immediately after the second subtracting circuit 40. The
first conversion circuit 72 includes a look-up table for converting
linear constellation points to non-linear constellation points. The
conversion is carried out by indexing into a non-linear
constellation table. A second converting means comprising a second
conversion circuit 73 is provided at the receive end 23 of the
communication channel 71, which comprises a reverse look-up table
which corresponds to the look-up table of the first conversion
circuit 72 for reconverting the non-linear constellation points to
linear constellation points. The signals from the second conversion
circuit 73 are then fed to the third adding circuit 41 where the
input data signal is recovered and outputted on the output 43.
Additionally, in this embodiment of the invention the difference
signal from the first subtracting circuit 35 is not added to the
input data signal, but rather, the difference signal from the first
subtracting circuit 35 and the input data signal are fed directly
to a selecting circuit which outputs a control signal of value kM
which is selected in response to the values of the input data
signal and the difference signal, and which in turn is fed to the
second subtracting circuit 40 and subtracted from the input data
signal. In this embodiment of the invention the value of k is
chosen for minimising the absolute value of the transmit signal
appearing at the node 30, rather than in the case of the precoding
circuit 20 where the value of k is selected for maintaining the
transmit signal at the node 30 within the range
.times..times. ##EQU00022##
While a multiplying circuit has only been described with reference
to the precoding circuit 50, it is envisaged that each of the
precoding circuits described with reference to FIGS. 4 to 8 will in
fact be provided with a multiplying circuit. It is also envisaged
that an all-pass phase corrector which has been referred to as
being provided with the precoding circuit 50 of FIG. 6 will be
provided in each of the other precoding circuits of FIGS. 4 and 5,
and FIGS. 7 and 8.
* * * * *